1. Field of the Invention
The present invention relates to an apparatus and process for shaping and tempering sheets of glass having a complex shape, for use as windows in vehicles.
The shape of the bodywork of a motor vehicle is of great importance, not only from an aesthetic point of view, but also from an aerodynamic one. In this context, it is also necessary that there be no surface discontinuity which would worsen the vehicle's air penetration coefficient when in movement.
For this reason glasses for automobiles in particular have taken on complex forms which can only be manufactured in specially produced plants built for the purpose.
2. Description of the Prior Art
Industrial plants for shaping and tempering sheets of glass with complex forms for use in motor vehicles are known from the state of the art.
U.S. Pat. No. 4,285,715 describes a plant which heats the glass, conveyed on rollers, in a horizontal oven and, again on rollers, transports it to a shaping station. A lower, vertically mobile mold whose flat surface is grooved so as to pass through the rollers transporting the glass, lifts the glass from the rollers and carries it upwards, until the glass is close enough to be attracted to the upper mold by means of vacuum suction. When shaping has been completed the upper mold allows the glass to drop onto a ring-like member, called a shuttle, running horizontally, which transports the glass to the tempering and cooling station.
The disadvantage in this plant lies mainly in the fact that the glass, which is at a high temperature, suffers surface deformation due to the long period of contact first with the rollers in the shaping area, and then with the flat surface of the mold. The surface deformation causes notable optical faults which are unacceptable for glass to be used in motor vehicles.
U.S. Pat. No. 4,508,556 describes a method and an apparatus for shaping sheets of glass having complex shapes. The sheets of glass move along a gaseous hearth bed through a horizontal heating oven, drawn by transversal bars separated by a given space, and reach the shaping station. In this area, a hearth formed by a gaseous bed takes up the glass and bears it in its horizontal movement. Because the hearth has a suitable downward curve, it causes the glass to become curved due to gravity into a desired shape. A hollow shaping ring, of the configuration desired, surrounds the gaseous bed and is positioned below the bed. When the whole surface of the glass is resting on the gaseous bed, the ring lifts it vertically towards a mold, to which the glass is attracted by vacuum suction, and which successively deposits the glass on a second shuttle carriage ring which carries it to the tempering and cooling station.
In this plant there are several disadvantages: first of all it is necessary to change, for production of each shape, not only the lower ring and the upper mold, but also the gaseous hearth bed, which has a curvature similar to that of the finished article; the gaseous hearth bed must also be changed in relation to the surface dimensions of the article, as it must be housed within the hollow shaping ring; and the transfer from the oven to the shaping station takes place by means of intermediate rollers which, given the high temperature of the glass, can easily cause surface deformation and therefore optical defects.
The European patent application EP 415,826 describes a plant which attempts to overcome the disadvantages deriving from the presence of rollers in the curving station, substituting the rollers with a belt of suitable heat-resistant material which is placed between a lower shaping ring and an upper vacuum mold.
In this case also there are certain disadvantages, deriving mainly from the difficulties in controlling such a complex shaping system, and from the fact that prolonged standing of the glass, heated to softening temperature, on the support belt can easily cause unpleasant optical deformations.